In a plant for producing urea, melamine, or the like, since ammonia, carbon dioxide, water, and the like are used in the plant, an aqueous ammonium carbamate solution is present as a recycled fluid of an unreacted material, a by-product, and/or a raw material in many cases. Accordingly, for operating such a plant, it is desired that the component composition of the aqueous ammonium carbamate solution be simultaneously measured quickly without a time lag with a simple apparatus.
Hereinafter, the outline of a urea production plant will be described using FIG. 4.
As shown in FIG. 4, urea is produced through the sections of synthesis section 31, decomposition section 32, concentration section 33, and finishing section 34. In the synthesis section 31, ammonia is allowed to react with carbon dioxide to synthesize urea to provide a urea synthesis solution. Unreacted ammonia and ammonium carbamate contained in the synthesis solution are separated as a mixed gas of ammonia, carbon dioxide, and water in the decomposition section 32. To absorption section 35, is fed water (condensed water separated in the concentration section 33 may be used) as an absorbent, and the mixed gas separated in the decomposition section is absorbed into the absorbent. For this absorption, an absorber (referred to as an unreacted gas absorber) is used. An outlet liquid from the absorption section (unreacted-gas absorber outlet liquid) is returned to the synthesis section 31 as a recovered liquid.
In the production plant of urea, it is desired that the loss of ammonia and carbon dioxide to the outside of the urea production plant be eliminated by allowing all the mixed gas separated in the decomposition section to be absorbed into the absorbent and recovering it into the synthesis section as an absorbent liquid. In order to allow all the mixed gas to be absorbed into the absorbent, the plant needs to be operated so that operating temperature is always kept lower than the equilibrium temperature. On the contrary, when the operating temperature is higher than the equilibrium temperature, it is impossible to absorb the whole mixed gas. The equilibrium temperature as described herein refers to a temperature at which the liquid composition at the time when the mixed gas to be absorbed is absorbed by an absorbent (water) is exactly in a vapor-liquid equilibrium state at a controlled operating pressure. The equilibrium temperature is determined by the concentrations of ammonia, carbon dioxide, and water.
For example, if the absorbent (water) is excessively fed to the absorption section, the equilibrium temperature will increase, which is advantageous to the absorption of the mixed gas. However, on the other hand, the excessive water increases the amount of water in the outlet recovered liquid from the absorption section to thereby increase the amount of water in the urea reactor (provided in the synthesis section) which receives the recovered liquid. Therefore, the urea synthesis rate in the urea synthetic reaction is reduced to increase an unreacted material in the urea synthesis solution. This results in a vicious cycle whereby the amount of heat required to remove the unreacted material is increased to increase the steam consumption in the urea plant, and also the amount of the absorbent (water) required to recover the separated unreacted gas is further increased. Therefore, it is important for the operation that the amount of water fed to the absorber in the absorption section be the requisite minimum. However, if the amount of water fed to the absorber is carelessly reduced, the equilibrium temperature of the outlet liquid from the absorption section may be lower than the operating temperature, thereby lowering absorption performance, which may cause the loss of ammonia and carbon dioxide. At this time, the operating temperature may be decreased in order to limit the amount of water fed to the absorber and improve the absorption performance. But, if the operating temperature is excessively reduced, the operating temperature may be lower than the solidification temperature (temperature at which ammonium carbamate cannot be dissolved in the recovered liquid but is precipitated as a crystalline salt) to solidify the recovered liquid, resulting in being impossible to continue operation. The solidification temperature is also determined by the concentrations of ammonia, carbon dioxide, and water. That is, it is desirable to always keep the operating temperature in the absorption section lower than the equilibrium temperature of the recovered liquid, to make the solidification temperature of the recovered liquid be higher, and to make the difference between the equilibrium temperature and the solidification temperature be small.
The equilibrium temperature and the solidification temperature of the recovered liquid are determined by the concentrations of three components of ammonia, carbon dioxide, and water, and are not determined only by the ratio of the amount of water to carbon dioxide, or the ratio of the amount of ammonia to carbon dioxide. In order to specify the equilibrium temperature and the solidification temperature of the recovered liquid, it is required to measure the concentrations of three components accurately and simultaneously without a time lag.
By the way, as a urea synthesis process, there is known a solution circulation process in which a urea synthesis solution from a synthesis reactor in the synthesis section is directly transferred to the decomposition section. Further, there is known a stripping process in which the urea synthesis solution from the synthesis reactor is transferred to a stripper in the synthesis section, and ammonia and carbon dioxide contained in the urea synthesis solution are stripped at a synthesis pressure using carbon dioxide as a stripping agent to be removed to certain concentrations.
Particularly in the stripping process, the ammonia concentration and the carbon dioxide concentration in the outlet liquid from the stripper used vary with the operating temperature of the stripper, the feed rate of carbon dioxide, the amount of feed liquid, and the like and influence the composition of the recovered liquid. That is, since the amounts of ammonia and carbon dioxide transferred to the unreacted gas absorber easily vary with the stripping performance of the stripper, it is difficult to control the amount of water fed to the unreacted gas absorber to the optimum amount in consideration of the equilibrium temperature and the solidification temperature of the aqueous ammonium carbamate solution present in the unreacted gas absorber. Therefore, in order to continue stable operation, the feed rate of water as an absorbent is generally increased somewhat to excess.
Accordingly, if the composition of the recovered liquid is simultaneously specified without a time lag, it will be possible to accurately find the equilibrium temperature and the solidification temperature of the recovered liquid (aqueous ammonium carbamate solution) from the resulting composition, thereby making it possible to determine an optimum operating temperature and to perform the operation with controlling the amount of water in the recovered liquid to a requisite minimum amount, in consideration of both the equilibrium temperature and the solidification temperature.
Various techniques have been proposed in order to analyze the physical properties of the unreacted-gas absorber outlet liquid.
Patent Literature 1 (JP6-184085A) discloses a method of measuring the electric conductivity of an unreacted-gas absorber outlet liquid to specify carbon dioxide concentration (ammonium carbamate concentration). However, this method cannot specify the concentrations of ammonia and water in a recovered liquid, and therefore cannot exactly determine an optimal point of operation.
Patent Literature 2 (JP59-133451A) discloses a method of specifying the concentrations of ammonia and carbon dioxide by determining density and a saturation temperature by means of an oscillation-type density meter and a photometer (measurement of crystal precipitation temperature). However, in this method, a photometer is used to measure a crystal precipitation temperature, and it is necessary to adjust the temperature of an unreacted-gas absorber outlet liquid to thereby cool the solution, in order to actually precipitate crystals from a sample of the unreacted-gas absorber outlet liquid. Such a procedure causes a time lag, and therefore this method is unsuitable for operation control.
Patent Literature 3 (U.S. Pat. No. 3,270,050A) proposes a method of keeping the concentration of an unreacted-gas absorber outlet liquid at a constant level by changing the amount of water fed as an absorbent using a viscometer, in a solution circulation process which is one of urea synthesis processes. However, this method only monitors the variation of concentration by use of viscosity and is not a method of specifying the composition of the unreacted-gas absorber outlet liquid. Further, the inventor of Patent Literature 3 himself admits in Patent Literature 4 (JP47-10226A) that viscosity is unsuitable for controlling the amount of water fed as an absorbent because there is influence of free ammonia in the method according to Patent Literature 3, and proposes in Patent Literature 4 a method in which a refractometer is used instead. Thus, it is clear that the composition of the three components cannot be specified by the method described in Patent Literature 3. Also from this point, it can be said that the method described in Patent Literature 3 cannot specify the concentrations. Furthermore, also in the method described in Patent Literature 4, only the concentration of ammonium carbamate is measured by a measurement with a refractometer, and similarly to Patent Literature 3, the whole composition of the unreacted-gas absorber outlet liquid cannot be specified.
Further, although a measuring object is different from the unreacted-gas absorber outlet liquid, Patent Literature 5 (JP58-90544A) discloses a method of specifying ammonium concentration by titration, carbon dioxide concentration by electric conductivity, and urea concentration by a colorimetric method with respect to the composition of a synthesis solution in a synthesis reactor. However, they are not different from the conventional manual analysis and require time for obtaining measurement results. Therefore, the method is unsuitable for operation control. The purpose of this method is adjustment of the amount of raw-material ammonia and carbon dioxide to be fed to the synthesis reactor, and cannot be used for optimization of the absorption section.
Patent Literature 6 (JP10-182586A) and Patent Literature 7 (JP2006-335653A) disclose a method in which the N/C ratio (ammonia/carbon dioxide ratio) of an outlet liquid from a synthesis reactor or an outlet liquid from a carbamate condenser is measured by density in a synthetic system of a stripping process. The methods described in these literatures specify the ratio of ammonia to carbon dioxide, the ammonia being total ammonia including urea, carbamic acid, and unreacted ammonia in a synthesis solution, and the composition of a synthesis solution cannot be specified. Further, the purpose of these methods is adjustment of the amounts of raw-material ammonia and carbon dioxide to be fed to the synthesis reactor, and these methods cannot be used for optimization of the absorption section.